bug#71784: 31.0.50; Inconsistent fontification for field_identifier in c++-ts-mode

[Ergus]

On Wed, Jun 26, 2024 at 06:46:04PM GMT, Eli Zaretskii wrote:
> > Date: Wed, 26 Jun 2024 16:13:47 +0200
> > From:  Ergus via "Bug reports for GNU Emacs,
> >  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> >
> > Using the c++-ts-mode I found that there is some inconsistent
> > fontification for the `fields_identifier`:
> >
> > See the fontification in this example with `emacs -Q`.
> >
> > ```test.cpp
> >
> > std::string key;
> > bool inserted;
> >
> > struct name_t {
> >         std::string key;
> >         bool inserted;
> > };
> >
> > name_t keys = {"aaa", true};
> >
> > keys.inserted = false;
> > bool a = keys.inserted;
> > ```
> >
> > 1. The `keys.inserted` values are shown differently before or after the
> > = (the inserted word is fontified is some cases, but not in all)
> >
> > 2. The variable names are fontified differently outside or
> > inside the struct.
> >
> > 3. The escape sequence (\t) is fontified differently to the rest of the
> > text inside the string. I don't know if that is intentional or not. If
> > it is intentional, just ignore this comment.
> >
> > The inconsistencies 1 and 2 are not only different to c++-mode but they
> > are semantically incorrect.
>
> What does treesit-explore-mode tell you about these instances of
> keys.inserted?

This is the whole explorer buffer for the example code:

(translation_unit
 (declaration
  type: (qualified_identifier scope: (namespace_identifier) :: name: 
(type_identifier))
  declarator: (identifier) ;)
 (declaration type: (primitive_type) declarator: (identifier) ;)
 (struct_specifier struct name: (type_identifier)
  body: 
   (field_declaration_list {
    (field_declaration
     type: (qualified_identifier scope: (namespace_identifier) :: name: 
(type_identifier))
     declarator: (field_identifier) ;)
    (field_declaration type: (primitive_type) declarator: (field_identifier) ;)
    }))
 ;
 (declaration type: (type_identifier)
  declarator: 
   (init_declarator declarator: (identifier) =
    value: 
     (initializer_list {
      (string_literal " (string_content) ")
      , (true) }))
  ;)
 (expression_statement
  (assignment_expression
   left: (field_expression argument: (identifier) operator: . field: 
(field_identifier))
   operator: = right: (false))
  ;)
 (declaration type: (primitive_type)
  declarator: 
   (init_declarator declarator: (identifier) =
    value: (field_expression argument: (identifier) operator: . field: 
(field_identifier)))
  ;))

The faces are:

1. Inside the struct insert has: font-lock-property-name-face
   It says `declarator: (field_identifier)` and I thin is applying the
   function c-ts-mode--fontify-declarator.

2. In `keys.inserted = false;` the `insert` words has: 
font-lock-property-use-face
   It says `field: (field_identifier)` and applies (I think) :feature 'property

3. In `bool a = keys.inserted;` is not fontified.
   But it says `field: (field_identifier)` like in 2.


Hope this helps.
Ergus